1. Field of the Invention
The present invention relates to audiological testing devices and specifically to an automatic diagnostic audiometer adapted to test the hearing of an individual wherein the individual being tested regulates whether the sound intensity to which he is being exposed automatically increases into or decreases from his range of audibility.
2. Description of the Prior Art
Automatic audiometry can be defined as a self-administered hearing test. More accurately, it is a hearing test performed by an instrument designed to present automatically changing tone frequencies while the degree of sound intensity of the signal is controlled by the examinee, the entire test sequence being simultaneously recorded on a synchronously coupled automatic recorder. The earliest known automatic audiometer was developed by Bekesy and improved by Reger. Reference is made to George von Bekesy, "A New Audiometer", Acta Otolaryngologica, Vol. 35 (1947), pages 411-422, and Scott N. Reger, "A Clinical and Research Version of the Bekesy Audiometer", Laryngoscope, Vol. 62 (December, 1952), pages 1333-1351. In accordance with the teachings of Bekesy, a motor driven pure tone oscillator is swept from the lowest to the highest test frequency in a continuous progression. In another mode of operation termed the "reverse Bekesy test", the oscillator is adapted to be swept from the highest to the lowest test frequency, likewise in a continuous progression. An attenuator or level control comprising, for example, a potentiometer, is driven by a reversible electric motor, the direction of which is determined by a push button switch operated by the examinee. The examinee is instructed to push the button as long as he hears the signal and keep it depressed until it fades from audibility, then to release it immediately. The tone will then fade into audibility again and the earlier process is repeated. The examinee then listens for the test tones through appropriate earphones. Upon his hearing the test tone and depressing the button, the motor causes the attenuator to decrease the intensity of the output through the earphones; when the button is released, the motor reverses itself and starts an increase in the intensity of the output signal. An ink writing recorder usually coupled by gears, chains, and the like, to the attenuating and frequency sweeping mechanisms of the audiometer, traces out an audiogram representing the examinee responses to the various test tones presented. Note, for example, U.S. Pat. No. 2,653,384 which teaches an apparatus embodying an automatic audiometer according to Bekesy, synchronously coupled to a drum recording mechanism. As a further reference, a representative automatic audiometer based on the above teachings of Bekesy was manufactured by Grason Stadler, Inc., of West Concord, Massachusetts, and was designated, during the period in which it was at one time manufactured, as Model E-800. This particular audiometer has found primary application in clinical diagnostic work and research.
An offshoot of the Bekesy clinical and research audiometer is the automatic screening audiometer widely used in industrial and military testing programs. The major difference between the Bekesy automatic and the screening automatic audiometers is that the latter uses discrete frequencies, usually 500, 1000, 2000, 3000, 4000, and 6000 Hertz, instead of the continuous frequency sweeping taught by Bekesy. The automatic screening audiometer in operation dwells on each of the above frequencies for approximately 30 seconds, automatically switches to the opposite ear and repeats each of the frequencies. During the 30 second test interval the examinee uses the manual pushbutton to trace his hearing threshold on a suitable chart or drum recording instrument. This type of audiometer is commonly referred to as the Rudmose Recording Audiometer. Reference is made to R. F. McMurray and Wayne Rudmose, "An Automatic Audiometer for Industrial Medicine", Noise Control, Vol. 2 (January, 1956), pages 33-36. A representative example of this type of audiometer is sold by Tracor Electronics Company of Austin, Texas, and is designated Model ARJ-4. Several other firms have also introduced new industrial automatic recording audiometers; for example, Medical Measurement Instruments, Inc., Model 1000 and Grason Stadler, Inc., Model 1703. Reference is also made to U.S. Pat. No. 2,781,416 which teaches an automatic screening audiometer. Other prior art to be considered includes U.S. Pat. Nos. 2,537,911; 2,781,416; 3,007,002; and 3,392,241.
While the above mentioned prior art has in every case constituted significant advances in the field of audiology, those skilled in the art have noted certain deficiencies over the years. The most commonly noted deficiency is the extraneous noise generated by the use of motors, gear drives, chain links, stepping switches, solenoids, and relays. Such devices are not only subject to wear and misalignment, but also are a source of acoustic noise, constituting a deterrent to the accurate determination of hearing thresholds. Additional and separate rooms of soundproof construction, costing, at present prices, approximately $100 per square foot, are usually required to contain the prior art type audiometers. Since the examinee is the operator of an automatic audiometer, however, it is desirable to have a silent audiometer capable of being left near the subject, or in any case, within the same room.
As an added disadvantage, audiometers of the prior art have employed vacuum tubes with their attendant failures and heat emission, often have weighed in excess of 100 pounds, and have occupied considerable table or console space. While newer updated versions of audiometers are lighter due to the partial utilization of semiconductor electronic components they still require approximately the same surface area for housing. Even those audiometers employing transistor construction, however, utilize numerous relays for switching, along with other electromechanical components. All existing automatic audiometers of the prior art have varying amounts of acoustic noise when received as new before wear on gear trains or motors occur.
Even further disadvantages have been cited by those skilled in the art regarding calibration of conventional audiometers. In most cases mechanical as well as electronic calibration parameters are involved and there is present the need to align the two with respect to each other. Calibration components are furthermore typically inaccessible when the conventional audiometer is in its normal operating mode. Even when access to calibration components is possible, components are not usually adjustable and must be desoldered and exchanged for other values.
Other audiometers of the prior art have employed photocells and appropriate variable light sources or field effect transistors, to induce variance in sound attenuation without requiring the use of the conventional potentiometric attenuators, but even these electronic components have not been wholly satisfactory in that they have introduced signal distortion and nonlinearity at some degrees of attenuation. Reference may be made to "Description of the Prior Art" referred to in U.S. Pat. No. 3,793,484, entitled "Programmable Audio Level Control Useful in Audiometric Apparatus" for an elaboration of some deficiencies.
A further disadvantage of instruments of the prior art relates to the necessity of assembling a large amount of ancillary equipment in order to implement various audiometric functions considered vital for clinical diagnosis. One example of such a function is the "lengthened off time" or "LOT" test which is later discussed and which in instruments of the prior art required the use of an external electronic switch of substantial size. Another example of such a function has to do with the provision of narrow-band tracking masking noise, as later discussed, for facilitating the determination of auditory thresholds in subjects having certain kinds of hearing deficiencies. Instruments of the prior art require an external electronic filter of substantial size and an additional source of wideband noise.
The additional equipment required to implement these two functions in addition to rendering portability of the equipment impossible, and to requiring substantial installation space, are research-oriented rather than user-oriented equipment. A high level of technical skill beyond the usual knowledge of the audiometric art was frequently required for the successful operation of this equipment.
Thus, it would be desirable to unify the above functions as well as other functions considered desirable into a single user-oriented instrument of reasonable size and weight which would preferably be portable. Such an instrument could be operated without requiring unusual technical skills and could in addition be carried to the bedside of test subjects who are unable to go to a testing area.
Improvements over the prior art are found in U.S. Pat. No. 3,793,484, entitled "Programmable Audio Level Control Useful in Audiometric Apparatus"; in U.S. Pat. No. 3,808,354, entitled "Computer Controlled Method and System for Audiometric Screening"; and in U.S. Pat. No. 3,793,485, entitled "Precision Automatic Audiometer", the specifications of which should be considered as incorporated by reference. The relation between this application and the referred to patents is, in a general sense, that U.S. Pat. No. 3,793,485 discloses a discrete frequency automatic audiometer useful in audiometric screening using a level control as described in U.S. Pat. No. 3,793,484, that the present application discloses a Bekesy type clinical or diagnostic audiometer using such a level control, and that U.S. Pat. No. 3,808,354 discloses a system using, via long distance communication, e.g., telephone lines, whereby a plurality of geographically widespread audiometers of a related type are modified for control and recording by a central computer whereby a plurality of test subjects at a plurality of geographically remote test sites may be tested simultaneously. Alternately, the computing means in U.S. Pat. No. 3,808,354 may be coupled locally to the audiometer and used on single or plural subjects.
The inherent deficiencies of conventional automatic audiometers in the prior art have also generated a need for an automatic clinical or diagnostic type audiometer having improved, more accurate performance, better reliability, having no moving component parts, and which is extremely quiet in operation. U.S. Pat. No. 3,793,485, entitled "Precision Automatic Audiometer" teaches a relatively quiet discrete frequency automatic audiometer for screening and what is needed is an improved diagnostic or clinical audiometer. Such an audiometer could now be placed in the immediate proximity of an examinee and by the elimination of massive conventional electro-mechanical components, could be enclosed in a small lightweight portable housing such as a portion of a chart recording instrument thereby occupying a minimum amount of table or console space. Also needed is an easily calibrated automatic clinical audiometer having pulse, sweep frequency and tone decay functions in one instrument.